Very unlikely that someone with authority will give you a definive answer. Best you will get is some vague line about how Itch does not like people trying to stretch the limit of what is sfw. I have seen similar threads.
If you have explicit nudity, there is no discussion, the game will be considered nsfw. If not by you, then by some users that will report your game. This is not a discussion about a region specific age rating. Also, you mentioned the game is in part about sex. So why is this even a thread.
Citing Outlast. Come on. https://www.esrb.org/ratings/10004839/outlast/
Outlast is rated M for Mature 17+ by the ESRB with Intense Violence, Blood and Gore, Sexual Content, Nudity, and Strong Language.
Sex sells. That is not a new insight. And using it to make a game appealing does not make the game artistically superior. Or the oil paintings one tries to sell for a living, because none want's to buy the boring things you painted.
One can do art and tell things without being graphically explicit. Espcially in a game where you control every pixel. So why do it. Maybe because the material is not for minors to begin with, like a gore horror intense violence game. Or maybe because, well, sex still sells. In movies it is often used as a shock value, or to show that this is for mature audience. Or as eye candy. Fan service. Rule 34 is a thing for a reason. Oh, and from the other side, adult games are still games. If people were in it for some quick recreational inspiration, there are other sites for that. But they are adults. Why play Mario, if they can play adult Mario. And why look at a landscape oil painting if you can look at something more exciting.